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Oh, it's too terrible! >:-{
He tells that he has used his unit for two months only and it was not like that when it was new! Indeed, in the same thread, there are users of K-7, K20D and K10D report the same, here, here and here, respectively.
So, this issue seems to be not uncommon. And the really worrisome thing is that it is found to be persistent since the first "Weather-Resistant" Pentax DSLR model K10D, back to 2007. The most strangest thing is that it happens mostly for those Pentax WR bodies, but rarely heard for non-WR entry-level Pentax DSLR bodies, on the other hand. Well, it seems that the weather-sealing is actually the culprit to this issue! E.g., when it was just not properly assembled out of the Pentax factory or when the sealing rubber and/or other materials are changing its/their state(s) over time?? :-o
Update (10-11):-
Thanks to Blog reader "calluses", we have the following yet another report from a Japanese user for exactly the same issue, also for a Silver K-5:-

(Source: http://d.hatena.ne.jp/hisway306/20110324/1300971454#seemore, in Japanese, Google English translation can be found here)
It is told that the above dust accumulated inside the LCD just from the very beginning and thus the user immediately asked for a direct replacement from the dealer and he got his replacement unit without any difficulty. It is also told that the replacement unit of his have not had the same issue after then.
Update (10-13):-
Yet here is another report of the same issue, for an ordinary black K-5:-
http://forum.xitek.com/showthread.php?threadid=924323
(in Chinese, English Here (Google-translated))
Of course, the user is pissed off. >:-(
bandymelis · 702 weeks ago
I also purchased the K5 for several months ago. Unfortunatelly, I have some small, barely seen scratches on the screen already (just because there were hard buttons on my shirt the other day). But not a speck of dust anywhere. Maybe, there were some batches of cameras that were not that successfull?
Daedbird · 702 weeks ago
Umm, RIce, a couple people complain, and you all of a sudden jump to the conclusion that this has been a problem since the K10? You, my sir, are insane. Look, I have had a K10 since it first came out. I have been out in torrential rains, I have been in the extreme cold, I have had my camera covered in mud. Not once have I gotten dust under any glass covering a screen. Your allegations are baseless (you have not direct contact with these complaints).
In every product line, there will be a few lemons, but this attack on the build quality of Pentax you have gone on is beyond the line. Stop sulking that you chipped your Kx's flash and Pentax wouldn't replace it....
Michael A. · 702 weeks ago
Have silver K-5 with extra hard tempered glass back and top, plus additional ist*D, K200D, K-x and black K-5 with NO dust under LCD. Makes one wonder what some users do with their gear to "find problems"....
BTW, that receding step, where the glass is seated in body, doesn't exist on my copy. It has a perfectly smooth and flat transition with all around deep black frame. Appears altered...
Sam Snork · 702 weeks ago
Ying · 702 weeks ago
This problem is dust actually getting INSIDE the camera body.
calluses · 702 weeks ago
http://d.hatena.ne.jp/hisway306/20110324/13009714...
Note that the dust would be under the LCD screen. He got a replacement eventually. Although many of his photos were taken with the 31mm (when he was using his first K-5), I noticed that he was using a few Pentax DA zooms and a huge SIGMA zoom as well. Not sure if the use of large zooms had anything to do with it.
I also vaguely remember a Pentaxforums member suffering the same issue. The theory had something to do with pumping in large amounts of air when using a large zoom lens...can't remember how it worked, but he tried to dust-proof his camera and was ridiculed for it.
Alexander Evensen · 702 weeks ago
Ying · 702 weeks ago
Michael A. · 701 weeks ago
RiceHigh 110p · 701 weeks ago
Chris · 702 weeks ago
Capelson · 701 weeks ago
Etienne
RiceHigh 110p · 701 weeks ago